Harvesting Pig Tissue for Transplants
March 3, 2014
Polish scientists have genetically modified several pigs so that their body parts can be harvested for use in human patients. The modified pigs have a combination of genes that makes them genetically closer to humans, the researchers say. As a result, the human immune system will be less likely to reject the animals’ body parts and organs during a transplant.
The researchers genetically modified the pigs under the “Transgenic Animals” project, which ran from 2010 to 2013. They removed the pigs’ antigens responsible for proteins that the human body particularly strongly defends against. Tissue grafted from these animals can be used to make, for example, bioprosthetic heart valves for human patients.
The project is expected to help enable scientists worldwide breed pigs whose body parts can used for human patients.
The “Transgenic Animals” project was managed by Dr. Piotr Wilczek from the Foundation for Cardiac Surgery Development in the southern Polish city of Zabrze. Poland’s National Center for Research and Development provided zl.3.5 million to help finance the project.
The Zabrze researchers—who worked together with colleagues from the Polish Academy of Sciences’ Institute of Human Genetics in Poznań and the Institute of Animal Production/National Research Institute in Cracow-Balice—have proved that genetic modifications make it possible to obtain excellent biological material for valvular bioprostheses.
Such bioprostheses are especially needed for children—doctors often have trouble finding donor organs suitable for small patients. With children, the problem does not end with a single operation. Their body grows, and after a few years, together with the changes taking place in the body, implanted valves can cease to function properly.
Other groups of patients have problems when their bioprostheses wear out. And the constant shortage of valve donors is an important limitation to the use of human tissue.
This is where tissue engineering steps in. The aim is to prepare animal tissue in such a way so that it can not only take over specific functions in the human body, but is not rejected by the human immune system.
Under a research project called “New methods for the preparation of biological valves,” Wilczek has developed methods for the removal of cells from animal material and for culturing the patient’s own cells on a special animal-derived matrix. This is how valves ready for transplantation were created.
While the elimination of cells with enzymatic and chemical methods will effectively remove most of the cells, it also carries the risk that some of the cells or their remnants will resist the process and become a source of antigens. They will cause an immune response from the human body and negatively affect the durability of the valves.
In the project, genetically modified animal tissue forms a matrix that is safe for the patient, a kind of scaffolding on which human cells are subsequently cultured. The pigs have the most critical antigens removed—those to which the human body could react.
Such a valve is safer for the patient, and the inflammatory response related to its implantation minimal.
The research is an attempt to find an alternative to efforts to breed animals for xeno-transplants, or the transplantation of living cells, tissues or organs from one species to another—a procedure that has proven to be a blind alley for medicine, according to the Zabrze researchers. There is growing skepticism among researchers about whether large organs in animals, such as the heart, lung and liver, can be modified in such a way so as to be fully suitable for transplantation into humans.
In the case of modified heart valves the situation is different. The Cracow team from the Institute of Animal Production/National Research Institute, led by Prof. Zdzisław Smor±g, bred several dozen animals in such a way that one of the animals’ critical antigens was removed. In turn, the Poznań group, led by Prof. Ryszard Słomski, checked if the modifications produced the desired result. Laboratory tests were carried out in Zabrze to determine whether the tissue engineering techniques worked well in the case of genetically modified animals. Previously developed methods concerned only non-modified animal tissue.
The main part of the work was an experiment on animals. The researchers wondered how the immunologically safer matrix would work in an animal model. The valves were built by covering the “scaffolding” of the porcine tissue with sheep cells. Then they implanted the valves in sheep to see to what extent the immune response associated with the presence of biological material derived from pigs could be suppressed.
Wilczek says, “We used pulmonary valves. This was a bit different than in the case of human patients, where we usually replace their own damaged valves with bioprosthetic valves. The animals’ valves were left in place, and the pulmonary valve was implanted in the descending aorta. This was easier than pulmonary implantation, which is more complicated surgically.”
Wilczek adds, “The load and pressure in the descending aorta are higher, so we knew that if our valve worked well in this environment, it would also properly function mechanically in the pulmonary position, and the risk of calcification would be limited.”
The research was conducted on 30 animals. In some sheep the researchers used commercially available valves, in others they implanted valves obtained as a result of various modifications. All the animals were expected to carry the implanted tissue for at least six months. The sheep survived and were healthy. After eight months, the scientists removed the valves and evaluated them in a laboratory. They checked the valves for any inflammatory infiltrates and calcification—calcification and calcium deposits make the valve unfit for further research, Wilczek says. In the target group, where the valves were covered with the sheep’s own cells, there was no calcification, and the tissue looked good.
The search for new ways of developing heart valve bioprostheses began with basic research. Functional tests for the bioprostheses will take another three years. If all goes well, the first clinical trials will then take place. Usually, in such cases, implantation of valves in five patients is allowed. If this small number of patients passes the clinical trials positively, the group will be increased.
The biggest problem is full commercial use of the new type of valves. Before a producer and distributor are found, a process of certification and accreditation is necessary for a product. However, the scientists say they are optimistic. Their research is part of the overall body of work being carried out around the world in this area. At the same time, the Polish research is unique in that it establishes an approach that no one else in the international scientific community has ever tried.
“R&D projects always contain an element of risk that the idea will not work,” Wilczek says . “So far we have been successful, and we will continue with our research. We will begin clinical trials. We’ll try to find funding for it.”